A mounting structure including a circuit board and a semiconductor chip such as an LSI mounted on the circuit board is manufactured by flip-chip mounting. In flip-chip mounting, projecting electrodes such as solder bumps are formed on the respective electrode terminals of a semiconductor chip, and then the semiconductor chip is mounted face down on the circuit board. Specifically, at least one of the semiconductor chip and the circuit board is heated; meanwhile, the projecting electrodes on the semiconductor chip are pressed to the respective electrode terminals of the circuit board.
Generally, a method of forming solder bumps includes the steps of: feeding solder onto the electrode terminals of a semiconductor chip by using a dispenser, screen printing, or electroplating; and heating the semiconductor chip at least to the melting point of the solder in a reflow furnace or the like. In the use of solder bumps, a space between the semiconductor chip and a circuit board is filled with molding resin so as to reinforce a bonding strength between the projecting electrodes on the semiconductor chip and electrode terminals on the circuit board.
In addition to solder bumps, projecting electrodes made of gold or copper are also available. Projecting electrodes made of gold or copper are formed by, for example, electroplating. In the use of projecting electrodes made of gold or copper, generally, an anisotropic conductive film containing an adhesive and metallic particles mixed in the adhesive is disposed between a semiconductor chip and a circuit board, and then the projecting electrodes on the semiconductor chip are pressed to electrode terminals on the circuit board.
Generally, electrode terminals on a semiconductor chip are arranged in a single row or are staggered in two rows on the outer end of the semiconductor chip. In order to obtain higher densities and higher pin counts on a semiconductor chip, a distance between electrode terminals has been shortened (smaller pitches) and an electrode area has been reduced. Pitches, in particular, have been considerably shortened. Thus, in the case where electrode terminals are arranged in one or two rows on the outer end of a semiconductor chip, a short circuit may occur between the electrode terminals or a faulty connection may occur depending upon a thermal stress caused by a difference in thermal expansion coefficient between the semiconductor chip and a circuit board.
Specifically, using solder bumps may cause a so-called a solder bridge leading to a short circuit between the electrode terminals. The solder bridge occurs when molten solder is deformed in flip-chip mounting so as to connect adjacent solder bumps with a solder surface tension. Thus, as the pitches of the electrode terminals decrease, solder bridges are more likely to occur. Moreover, a faulty connection occurs depending upon a thermal stress caused by a difference in thermal expansion coefficient between the semiconductor chip and the circuit board when molding resin to be fed into a gap between the semiconductor chip and the circuit board does not spread over the gap and only fills a part of the gap. Thus, as the pitches of the electrode terminals decrease, a flow of molding resin may be blocked so as to increase the possibility of the faulty connections caused by a thermal stress.
As has been discussed, as a distance between the electrode terminals decreases (smaller pitches), a short circuit may occur between the electrode terminals or a faulty connection may occur depending upon a thermal stress caused by a difference in thermal expansion coefficient between the semiconductor chip and the circuit board. Thus, in order to increase a distance between the electrode terminals, the electrode terminals are arranged in a matrix (grid pattern) in the plane (in the area) of the semiconductor chip. Electrode terminals arranged in a matrix (grid pattern) in the plane of a semiconductor chip have a larger layout area than electrode terminals arranged in one or two rows only on the outer end of the semiconductor chip, thereby increasing a distance between the electrode terminals.
In recent years, however, a distance has been considerably reduced between electrode terminals (smaller pitches) arranged in a matrix (grid pattern) on a semiconductor chip. Thus, also in the case where the electrode terminals are arranged in a matrix (grid pattern) in the plane of the semiconductor chip, short circuits are likely to occur between the electrode terminals.
In order to solve the problem, for example, Japanese Patent Laid-Open No. 9-97791 proposes a projecting electrode having a two-layer structure in which an upper solder electrode is formed on a high-melting lower electrode that does not melt during flip-chip mounting. The projecting electrode requires a smaller amount of solder than a solder bump including only solder, and reduces the amount of solder projection in a planar direction during flip-chip mounting, thereby reducing the occurrence of solder bridges.